Answer: In my experience a double walled panel be more energy efficient than plain plastic sheeting, so depending on your heating costs and the temperature you need to maintain, you may want to reconsider. Another variable would be whether or not it is a leanto attached to your house or a freestanding structure, the lean to is easier to heat and can take advantage of the larger structure's ability to hold heat. Such a small greenhouse will have fast temperature swings so you may also need to provide for automatic venting on sunny days. If you are using it more as an unheated cold frame for hardening off seedlings and transplants rather than as a place to grow plants all winter, an inexpensive but sturdy plastic sheeting may be fine as long as you are prepared to replace it fairly often. Here is some general information you may find helpful in planning your materials and design.
Have fun with your greenhouse!
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